Butyl rubber (copolymers of isobutylene with up to 20 wt % isoprene) and halogenated butyl rubber are often the materials of choice for use as sealing barriers for pharmaceutical containers. Such sealing barriers include stoppers, diaphragms, gaskets and pistons for use with vials, cartridges, syringes and the like. Butyl rubber exhibits superior properties in terms of air and moisture impermeability, resistance to chemical attack and heat resistance, as well as good processability, which are all desirable properties for pharmaceutical packaging.
One of the more important applications for butyl rubber is its use as a rubber stopper in glass or high density plastic vials. This affords relatively safe storage of medicines and easy access to the medicine by piercing the stopper with a needle cannula, removal of the vial contents and removal of the needle cannula from the stopper after which the vial is resealed due to the resilient nature of the rubber.
In other applications molded butyl rubber parts are used as components of plungers or pistons in syringes used to inject medicine into the body.
Of critical importance with respect to these uses is that the rubber must not only be resistant to chemical attack by the contents of the container with which it comes in contact, but it must also not contaminate the container contents by virtue of a leaching out of ingredients present in the cured rubber part. In the case of butyl rubber and other rubbers used for these applications, contamination sources would include vulcanizing agents required to cure the rubber, such as zinc or magnesium oxide, sulfur, sulfur-containing vulcanization accelerators, fatty acids, peroxides and the like. Some of these sources may contaminate certain medicines either directly or by catalyzing unwanted reactions in the medicine.
Compositions including copolymers of vinyl aromatic monomers have been disclosed as useful for some sealing applications, such as in WO 99 31178; WO 00 52072; and EP 0 474 406 A2. However, it has not been shown that these copolymers are useful as pharmaceutical seals, where avoidance of contamination is desirable. What is needed is an improved composition having excellent sealing properties, while avoiding contamination of stored materials.